Corrosion inhibiting composition



Patented Apr. 21, 1953 UITED STATES PTENT OFFICE CORROSION INHIBITING COMPOSITION Delaware Application May 21, 1951, Serial No. 227,352

7 Claims. 1

This invention relates to a corrosion inhibitor for use in inhibiting the corrosion of ferrous metal piping and tubing in producing oil wells and in pipe lines transporting crude oil. More particularly, it relates to a solid arsenous corrosion inhibitor.

The corrosion of ferrous metal tubing and piping in producing oil wells and in pipe lines transporting crude oil is now and has for some years been a serious operating problem confronting the petroleum producer. Corrosion of tubing and piping necessitates frequent interruptions of production to pull corroded tubing and piping from the well and replace them with new material. The essential features of a corrosive environment very commonly encountered in producing oil wells include a ferrous metal in contact with oil, brine, and gas containing carbon dioxide, gas liquid interfaces in contact with the metal, and areas of turbulent liquid flow in contact with the metal surfaces. The metal in this environment is corroded away by direct attack of carbonic acid which is greatly accelerated by electrochemical phenomena arising out of the contact of the phase interfaces with the metal and out of the contact of turbulent flowing liquid with the metal surface. Substantially all of the acidity of the production stream stems from the presence of carbon dioxide. The acidity is quite low, the pH not being lower than 3 and being ordinarily in the range 4 to 6. When a ferrous metal is exposed to the production eiiluent under quiescent conditions where there is no movement on the liquid or of free gas bubbles through the liquid, the corrosion rate observed is usually less than about 25% of that observed under actual producing conditions which include turbulent motion of the liquid in contact with the metal and the presence of minute gas bubbles which contact the metal. In laboratory apparatus in which the separate influence of the three corrosive factors was studied, i. e., direct acid attack, electrochemical attack attributable to turbulent liquid flow, and electrochemical attack attributable to the presence of finely-divided gas bubbles which contact the metal surfaces, it was found that the relative rate of corrosion due to direct acid attack was 4 units, and that the relative rate of corrosion when either the condition of liquid turbulence or finely-divided gas bubbles moving through the liquid was superimposed on the presence of the acid medium, the relative corrosion rate immediately increased to 14 units. From these observations it is clear that the corrosion problem would not be solved by the mployment of an inhibitor which would eliminate only the corrosive action due to direct acid attack and that the corrosion problem here presented can be solved only if the additional electrochemical corrosion attributable largely to physical conditions is markedly reduced. The corrosion inhibitor of this invention is especially well adapted to minimizing the corrosion of ferrous metals in the environment and by the mechanisms described.

A variety of physical setups is found in producing oil wells which determine the manner of introduction of the corrosion inhibitor into the well. Some wells have open annuli and a corrosion inhibiting solution can be lubricated into the well through the open annulus. Some wells have packed-off annuli and the corrosion inhibitor must be introduced through the production tubing by pumping an inhibitor solution to the well bottom and permitting it to flow upward with the production stream or by dropping a solid corrosion inhibitor through the tubing to the well bottom and producing the well. In condensate wells, where there is no liquid production stream, the corrosion inhibitor to be effective must in some manner be applied to the interior surface of the tubing without the aid of a liquid production stream to carry and spread it.

A solid corrosion inhibitor which can be placed at or near the well bottom by dropping it through the well tubing can be utilized effectively in all types of wells if the solid inhibitor has the following properties: First, it must be an effective inhibitor at low concentrations. Only relatively small amounts of an inhibitor can be introduced into a well by dropping the solid inhibitor through the tubing. Some commonly used inhibiting chemicals such as dichromates and caustic soda will inhibit satisfactorily if used in large amounts, but these materials are not suitable for injection into the well in solid form since the maintenance of the desired concentration requires very frequent interruption of the production for the introduction of further amounts of the inhibitor. A suitable chemical inhibitor for injection into the well through the tubing in solid form would be one that provides protection against corrosion at concentrations of about one to five parts per million in theproduced water; second, an effective solid corrosion inhibitor should have a high persistency in its inhibiting eifect. To be practical, the injections of a solid inhibitor should not be made more often than once in twenty-four hours, and preferably once in several days, or once a week. High persistency of the inhibitor or inhibiting film formed on the metal surfaces of the tubing is highly desirable in solid inhibitor injection. Third, a' succesful solid corrosion inhibitor should be highly soluble in well water. This is particularly desirable if the inhibitor is to be used in a condensate well. Relative insoluble or highly insoluble materials could easily cause plugging of the tubing. Fourth, a solid corrosion inhibitor should have a high density so that the solid inhibitor will fall through the production stream while the well is being produced. If the solid inhibitor has this property, theproduction need be interrupted for only a few minutes necessary to introduce the solid into the tubing. Fifth, the solid inhibitor, if it is to be efiective in a condensate well, should have a low melting or softening temperature. Condensate wells produce little or no formation water and much of a water-soluble chemical that falls below the condensate zone will not be returned with the production, ,If the "solid inhibitor softens or melts at temperatures from about 130 to 180 F., the solid will be spread on the hot tubing wall as it falls in the well and will give good distribution of the inhibitor over the interior of the tubing;

Arsenous compounds, particularly the alkali metal arsenites or slurries of arsenous oxide with alkali metal hydroxides and water, meet the above-described requirements with respect to effectiveness at low concentration, persistence, and solubility admirably. Particular arsenous compositions hereinafter described meet not only the first three requirements, but also the requirements with respect to density and to softening point in a remarkable manner.

It has been found that a hard dense homogeneous solid corrosion inhibitor can be prepared by intimately mixing arsenous oxide with an aqueous solution of sodium hydroxide and cooling the resultant mixture to remove exothermic heat of reaction. The mixture contains from 1.5 to 3.5 parts by weight of water to parts by weight of sodium hydroxide and arsenous oxide together and the ratio of arsenous oxide to sodium .hydroxide in the mixture is in the range 1:1 to 2:1, and preferably in the range 1.1:1 to 1.5:1.

Compositions containing these ingredients in these proportions are extremely hard and durable at ordinary temperatures, have a high density above about 3.0 grams per cubic centimeter and are thermoplastic in the sense that they become soft at temperatures in the range 110 to 130 F. and finally become liquid at temperatures above about 140 F.

The properties of numerous compositions consisting essentially of arsenous oxide, sodium hydroxide and water have been studied. Observations of these compositions are reported in tables of data hereinafter and some of the observations are graphically represented in the appended drawings of which Figure 1 is a graphical representation of the variation in hardness of the compositions as the proportions of arsenous oxide and sodium hydroxide are varied while holding the water content constant, and Figure 2 is a graphical representation of the variation of the properties of the compositions as the proportions Of the three components, arsenous oxide, sodium hydroxide and water, arevaried.

The results of a number of experiments in which the setting characteristics of varying mixtures of arsenous oxide, sodium. hydroxide and .4 water are set forth in the following Table I. Small batches of these chemicals were mixed using different proportions of the materials in each mixture. The mixtures were most readily made by dissolving sodium hydroxide in water and then adding arsenous oxide to the solution with rapid stirring. A considerable heat of reaction was evolved upon the addition of the arsenous oxide, the temperature rising to about F. and the mixtures becoming highly fluid and homogeneous. After standing at room temperature for 24hours, the mixtures were examined for hardness and rated as hard, soft or liquid on an arbitrary scale. The rating hard was applied where the material could not be distorted by manual pressure, and the rating soft was applied where the sample could be deformed by manual pressure.

TABLE I Set consistency of difierent Aszos-NaOH-Hzo mixtures Grams N'aOH per Grams AS20 GlaIlS 2 1 1% S S *S "H *S "H 2 S S S H S H 2% S S S H S H 3 S S S H S H 3% S L S H S H 4 S L S H *"S H '1 Too little water for mixing.

bgtlglegessary to add extra water for complete mixing in small *No ri-homogeneous mixtures-2 phases form-aqueous NaOH solid-hardness data is for solid.

' From the table it can be seen that those mixtures in which the proportion of water ranged from 1.5 to 3.5 parts by weight hardness was encountered at two ratios of sodium hydroxide to arsenous oxide, that is, at the ratio of 7:8 where these materials were present in approximately equal amounts and at the ratio of 11:4 where the amount of sodium hydroxide considerably exceeded the amount of arsenous oxide.

In conducting the experiment summarized in Table I, it was found that approximately 1 /2 parts by weight of water to a total of 15 parts by weight of sodium hydroxide and arsenous oxide together appeared to be about the minimum amount of water with which a homogeneous mixture could be prepared. Somewhat smaller amounts of water can probably be employed and a satisfactory degree of homogeneity achieved with more vigorous mixing. In this series of experi ments it was also observed that when the ratio of sodium hydroxide to arsenous oxide ranged from 5:10 to 9:6, 3 /2 parts by weight of water to 15 parts by weight of combined arsenous oxide and sodium hydroxide was the maximum amount of water which could be worked into the mixture to form a single phase homogeneous mixture, and that if larger amounts of water were employed the mixture separated into two phases, a lower opaque phase having a high solid content and an upper translucent phase consisting principally of water and sodium hydroxide. Upon cooling this two-phase system both phases solidified but the solid was non-homogeneous and consisted of two solid phases of difierent physical properties which did, however, adhere to each other.

A further series of mixtures was prepared to determine more accurately the efiect of varying the proportion of arsenous oxide and sodium hydroxide in the mixtures. The results of these tests are graphically represented in Figure 1 of the appended drawings. In all of the mixtures whose properties are shown in Figure 1, two parts by weight of water were mixed with a total of 15 parts by weight of arsenous oxide and sodium hydroxide. After the mixtures were prepared they were permitted to stand at room temperature until cool and then were tested for hardness and assigned an arbitrary hardness classification and diiferent amounts of water in each mixture. The mixtures were allowed to stand at room temperature for 24 hours and were then classified for hardness using the probe test previously described. It was found that there was a maximum water content at which it was possible to produce single phase homogeneous mixtures. The results of this series of experiments together with remarks on the homogeneity of the mixtures are by means of a simple probe test. A pointed metal 1 shown in the following Table II.

TABLE II Effect of water on hardness and homogeneity of mixtures containing 8 grams of A8203 and 6 grams of NaOH rod, /8 inch in diameter, was pressed into the surface of each mixture and then removed. The Hardness classifications used are as follows:

Paste.-The rod entered the mixture easily and the hole tended to fill when the rod was removed.

CZay.--The rod entered the mixture easily but the hole maintained its shape when the rod was removed.

H ard.The rod could be forced into the surface but not much beyond the sharp point.

Very Harri-The rod could not be forced into the surface Without chipping the solid.

Hardness classifications were made by two different observers independently and the results were plotted as shown in Figure 1.

Figure 1 shows a rapid change in the hardness of the mixtures for small changes in the proportion of arsenous oxide to sodium hydroxide. Two ranges of hard set mixtures are shown in Figure 1. To produce the first series of hard set mixtures the ratio of arsenous oxide to sodium hydroxide is in range 1:1 to 2:1, the optimum hardness being attained at arsenous oxide to sodium hydroxide ratios in the range 1.1:1 to 1.5:1.

A second series of hard setting mixtures is shown by Figure 1 to be formed when the ratio of arsenous oxide to sodium hydroxide is 1:2 and lower.

The first series of hard setting mixtures of arsenous oxide, sodium hydroxide and water is preferred for use as a solid corrosion inhibitor because of its higher content of arsenous material which is the primary corrosion inhibiting component of the mixture. The first series of hardsetting mixtures has a further advantage over the second series in that it is considerably denser, having a density above about 3.0 grams per cubic centimeter and an appreciably lower softening point which renders it more satisfactory for use in condensate wells.

A series of experiments was conducted to determine the effect of water on a hard setting of an arsenous oxide-sodium hydroxide composition in the optimum range. A series of mixtures were made using 6% parts by weight of sodium hydroxide, 8 parts by weight of arsenous oxide The data summarized in Table II indicates a practical upper limit to the amount of water which may be used in making the mixtures of the invention. Increased amounts of water gradually soften the total mixture and, in addition, when the amount of water used exceeds 3 /2 grams of water for 15 grams of combined arsenous oxide and sodium hydroxide the mixtures separate into two layers. The upper layers of the separated mixtures were found to contain a high concentration of sodium hydroxide, and most of the arsenous oxide was in the lower layers. The hardness data applies to the lower layer which was allowed to set after the upper layer had been decanted from the mixture. The low sodium hydroxide content of the lower layers had the effect of considerably reducing the solubility of the solidified mixture, a characteristic which is generally undesirable when the solid is employed as a corrosion inhibitor. The amount of water which should be employed in making the mixtures of the invention ranges from the minimum amount sufficient to produce a fairly homogeneous mixture, that is, 1 to 1 /2 parts by weight to a total of 15 parts by weight of arsenous oxide and sodium hydroxide up to a maximum value of 3 A; parts by weight of water per 15 parts by weight of combined arsenous oxide and sodium hydroxide.

Data showing the effect of temperature on the two types of hard seting compositions shown in Figure 1 is summarized in the following Table III. The compositions tested had five different proportions of arsenous oxide to sodium hydroxide, and with one of these compositions the proportion of water was varied. Test tubes containing the samples were placed in an oil bath and allowed to stand at each indicated temperature for a period of 30 minutes which was ascer tained to be long enough to bring the contents of the test tube to oil bath temperature. At the first indication of softening the samples were classified as slightly soft, at definite softening they were classified as soft, and if they became fluid they were classified as liquid.

Composition Temperature,- F.

. 7 2 fl i a Range AS203, N aOH, Parts by Partsby' Parts by 90 95 110 130 140 150 170 190 200 .Wt. Wt.

I 3% 6% 2 H H s1.s s s Narrow Ccntl'al Range 8% 6% 2% H H S1. S S L v 8%,, 6% 3%. H H s1.s s L l 1O 2% H H H S1. S S L L L L High NaOH Concentration '4 ll 2% H H H H H S s s 3 Range 3 12 H H H H H S]. S S S S 2 13 2% H H H H H S1. S S S S I 14 2% H H H H H S]. S L L L The data summarizedin above Table III shows that the softening temperatures of the The setting characteristics of different. mixtures of arsenous oxide, sodium hydroxide and water are generally summarized in graphical form in Figure z of the drawings. The two clear areas mEigure 2 indicate hard homogeneous mixtures of arsenous oxide, sodium hydroxide and water. The larger of these two areas on the right-handside of: the drawing represents operable mixtures, but mixtures having a somewhat lowerdensity, higher softening point, and considerably lower content of the active arsenous component of the, corrosion inhibitor than the compositions represented by the smaller clear area in the central portion; of the figure. It is the latter compositions with which the present invention is concerned. These compositions are prepared as indicated above by stirring arsenous oxide in an amount within the range indicated in the figure into an aqueous solution of sodium hydroxide containing these two ingredients in the proportions indicated in therfigure- The re sulting mixture is very hot and highly fluid and can be poured into molds of any desired shape and allowed 'to solidify in the molds. The resultant solids are macroscopically homogeneous andihave. a high densityabove about 3 grams percubic centimeter and a relatively low softening point around; 130 F}. These compositions can be'heated untilthey become liquid and re solidified'without afiecting their physical properties or homogeneity. This characteristic makes it possible to prepare large solid blocks of a composition which may be transported some distance from the point of manufacture, if desired, and theremelted and cast into any desired form.

The corrosion-inhibiting compositions of this invention are ordinarily'cast in the form of a cylindrical stick from 1 to 3 feet in length and having a diameter from lto 2 inchesthat is, a diameter generally adapted to permit the fall of the stick through conventional well tubing. The sticks ordinarily weigh from 3 to 9 pounds and have a density above about B grams per cubic centimeter. Their mass anddensity are such that they readily fall through the production stream of theiave'rage producing well so that the-production-need be interrupted only for the, fewminutes-whichare: required to introduce the stick into the well tubing. The introduction of such a stick every 3 to 10 days will ordinarily bring the corrosion rate of a well down to an acceptably low level and maintain it there.

Since arsenical compounds are poisonous, the sticks are desirably wrapped or coated with some non-toxic material in the interest of safe handling. The wrapping or coating may be either functional or non-functional and may be an integral part of the stick as it is dropped into the well, or may be removable at the time of the injection.

.Metals having a standard oxidation reduction potential above 0.5 and below 2.5 volts, such as magnesium, aluminum, zinc, and their alloys, form excellent functional coatings for the inhibitor. Corrosion inhibiting cartridges can be prepared by pouring the molten corrosion inhibitor into these metallic tubes and permitting it to solidify there. The bottom of the tube is closed to prevent the dissolving of' the inhibitor from that end of the tube in the well. The bottom then can be closed off by capping with a metallic cap, or crimping, or sealing with a. high melting point asphalt. If an asphalt plug is used in the bottom of the metal tube, the asphalt is selected to have a melting point above the bottom hole temperature of the well. Such a plug is removed by gradual dissolving of the asphalt in the oil phase. This process requires a sufiicient periodof time that most of the inhibitor goes into solution from the top of the tube. The top of the tube is ordinarily sealed with a high melting point wax, for example a wax melting from about F. to F. The metallic coating is an integral part of the inhibitor cartridge as it is dropped into the Well and the metal is functionalin its action in the well and not merely a container for the inhibitor. Magnesium, for example, slowly dissolves in dilute carbonic acid contained in the well water and, in dissolving, neutralizes apart of the acid; in addition, metals such as magnesium, zinc, aluminum and their alloys provide cathodic protection against electrolytic corrosion.

Other integral coatings which are non-functional may be employed such as a high melting point wax or an oil-soluble or water-soluble plastic material. The solidified inhibitor stick can be dipped in these materials and allowed to dry and the surface film of these materials provides protection to personnel against handling hazards. In the-well the protective coating is rapidly dissolved, placing the inhibitor composition in contact with the production stream.

The inhibitor sticks may also be protected by an outer container which is removed prior to the introduction of the inhibitor stick into the well. The hot, soft corrosion inhibitor ispoured into paper cylinders. which maintain their shapes when the temperature of the inhibitor reaches or exceeds its softening point during shipment or storage. The paper tubing is peeled away from the stick before dropping it into the well tubing. It has been found that if the hot corrosion inhibitor mixture is poured into untreated paper tubes, some difliculty is experienced in attempting to remove the paper from the hardened stick. However, if the paper is greased with a fairly heavy grease, or covered with a water-impervious coating such as shellac or plastic, prior to the introduction of the molten inhibitor composition into the paper tube, the paper may easily be removed from the solidified stick.

The corrosion inhibitor of this invention was tested in a producing well. The well was 9,000 feet deep and produced 353 barrels of oil, 210 barrels of salt water and 3,000,000 cubic feet of gas daily. The gas contained 2% by weight of carbon dioxide. The corrosion inhibitor was prepared by mixing 2 parts by weight of water with 5 parts by weight of sodium hydroxide and parts by weight of arsenous oxide. The molten mixture was poured into a magnesium tube 3 feet long and 1% inches in diameter. The tube was pinched off at one end and coated with 125 melting point wax at the other end. The stick was introduced into the well by placing it above the master valve in the Christmas tree. The hammer union at the bottom of the Christmas tree was closed and then the master valve was opened. The stick could be heard falling down the tube at high velocity. A restricting device had been installed below the stringer in the tubing prior to the introduction of the stick into the tubing so that it could not fall beyond the tubing string.

The effectiveness of the corrosion inhibitor to inhibit corrosion of ferrous metal tubing was determined by observing the iron count of produced water before and during the treatment. The iron count, a measure of the corrosion rate, is the number of parts per million of iron calculated as ferric oxide contained in the produced water as determined by the thiocyanate colorometric method (Scotts Standard Method of Chemical Analysis, 5th Edition, Van Nostrand, 1939, page 486). During the test, one inhibitor stick was introduced into the well each day during the first five days of the test, after which one stick was introduced on alternate days. During the test, the iron count pickup of the production water was reduced from an average value of 19 prior to the commencement of the test to an average of 3 during the continuance of the test. This reduction represents the elimination of 84% of the corrosion normally sustained.

The effectiveness of the inhibitor in a pipe line carrying the crude production stream was tested by suspending iron coupons in the pipe line and determining the corrosion rate by determining the weight loss of the coupons. Prior to introducing the inhibitor into the line the corrosion rate was determined and found to be 35.5 mils penetration per year. In a 13-day test in which the solid inhibitor of this invention was employed, the corrosion rate was found to be 5.5 mils penetration per year; the results of the coupon tests indicate 84% reduction in corrosion.

We claim:

1. A hard, dense, macroscopically homogeneous solid prepared by intimately mixing arsenous oxide with an aqueous solution of sodium hydroxide and cooling the resultant mixture to remove exothermic heat of reaction, said mixture being 10 formed from 1.5 to 3.5 parts by weight of water to 15 parts by weight of sodium hydroxide and arsenous oxide together and containing arsenous oxide and sodium hydroxide, respectively, at a weight ratio in the range 1 1 to 2: 1.

2. A hard, dense, homogeneous solid prepared by intimately mixing arsenous oxide with an aqueous solution of sodium hydroxide and cooling the resultant mixture to remove exothermic heat of reaction, said mixture being formed from 1.5 to 3.5 parts by weight of water to 15 parts by weight of sodium hydroxide and arsenous oxide together and containing arsenous oxide and sodium hydroxide, respectively, at a weight ratio in the range 1.1:1 to 1.5:1.

3. A hard, dense solid comprising the reaction product of arsenous oxide, sodium hydroxide and water, the ratio of arsenous oxide to sodium hydroxide being in the range 2:1 to 1:1 and the water content being at least one part by weight and not more than 3.5 parts by weight to each 15 parts by weight of arsenous oxide plus sodium hydroxide.

4. A hard, dense solid comprising an intimate mixture of sodium hydroxide and arsenous oxide with water, said mixture being formed from 1.5 to 3.5 parts by weight of water and 15 parts by weight of sodium hydroxide and arsenous oxide together and having arsenous oxide and sodium hydroxide present at a weight ratio in the range 1:1 to 2:1.

5. A hard, dense solid comprising an intimate mixture of sodium hydroxide and arsenous oxide with water, said mixture being formed from 1.5 to 3.5 parts by Weight of water and 15 parts by weight of sodium hydroxide and arsenous oxide together and having arsenous oxide and sodium hydroxide present at a weight ratio in the range 1.1:1 to 15:1.

6. A hard, dense thermoplastic solid formed by slurrying arsenous oxide in an aqueous sodium hydroxide solution and cooling the resulting slurry to remove the exothermic heat of reaction whereby the slurry sets, the proportions of the components of the surry being 1.5 to 3.5 parts by weight of water to 15 parts by weight of arsenous oxide and sodium hydroxide and the weight ratio of arsenous oxide to sodium hydroxide in the slurry being in the range 1:1 to 2:1.

7. The method of inhibiting corrosion in at producing oil Well delivering a production stream comprising crude oil, brine, and carbon dioxide gas which comprises periodically interrupting the production, dropping a solid corrosion inhibitor formed by intimately mixing arsenous oxide and sodium hydroxide with water, said mixture containing from 1.5 to 3.5 parts by weight of water to 15 parts by weight of arsenous oxide and sodium hydroxide together and having arsenous oxide and sodium hydroxide present at a weight ratio in the range 1:1 to 2:1, through the well tubing to the bottom of the well and producing the well.

GILSON H. ROHRBACK. DWITE M. McCLOUD.

WILLARD R. SCOTT.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,638,710 Sherbino Aug. 9, 1927 1,877,504 Grebe Sept. 13, 1932 2,396,465 Karr Mar. 12, 1946 2,546,586 Cross Mar. 27, 1951 

1. A HARD, DENSE, MACROSCOPICALLY HOMOGENEOUS SOLID PREPARED BY INTIMATELY MIXING ARSENOUS OXIDE WITH AN AQUEOUS SOLUTION OF SODIUM HYDROXIDE AND COOLING THE RESULTANT MIXTURE BEING EXOTHERMIC HEAT OF REACTION, SAID MIXTURE BEING FORMED FROM 1.5 TO 3.5 PARTS BY WEIGHT OF WATER TO 15 PARTS BY WEIGHT OF SODIUM HYDROXIDE AND ARSENOUS OXIDE TOGETHER AND CONTAINING ARSENOUS OXIDE AND SODIUM HYDROXIDE, RESPECTIVELY, AT A WEIGHT RATIO IN THE RANGE 1:1 TO 2:1. 